Having trouble creating an Addon with multiple modules
The add-on registration can be simplified using a register() and unregister() function per file that can be called by the __init__.py. You've already implemented these to test your operators, but they can be used for the full add-on as well. Therefore all you have to do is import the kitfoxStairs.py and kitfoxStairsCurved.py and then call their register() function in the __init__.py.
# Copyright 2019 Mark McKay
#
# Licensed under the Apache License, Version 2.0 (the &quot;License&quot;);
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an &quot;AS IS&quot; BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.


bl_info = {
    &quot;name&quot;: &quot;Curved Stairs Mesh Generator&quot;,
    &quot;description&quot;: &quot;Adds new mesh builders for quickly creating staircases.&quot;,
    &quot;author&quot;: &quot;Mark McKay&quot;,
    &quot;version&quot;: (1, 0),
    &quot;blender&quot;: (2, 80, 0),
    &quot;location&quot;: &quot;View3D &gt; Add &gt; Mesh&quot;,
    &quot;wiki_url&quot;: &quot;https://github.com/blackears/blenderStairs&quot;,
    &quot;tracker_url&quot;: &quot;https://github.com/blackears/blenderStairs&quot;,
    &quot;support&quot;: &quot;COMMUNITY&quot;,
    &quot;category&quot;: &quot;Add Mesh&quot;
}


import bpy
from .operators import kitfoxStairs
from .operators import kitfoxStairsCurved


def register():
    kitfoxStairs.register()
    kitfoxStairsCurved.register()


def unregister():
    kitfoxStairs.unregister()
    kitfoxStairsCurved.unregister()

If you want to reload modifications of your scripts when they have been imported before, then you need to replace the import statements
import bpy
from .operators import kitfoxStairs
from .operators import kitfoxStairsCurved

with the following code:
if &quot;bpy&quot; in locals():
    import importlib
    if &quot;kitfoxStairs&quot; in locals():
        importlib.reload(kitfoxStairs)
    if &quot;kitfoxStairsCurved&quot; in locals():
        importlib.reload(kitfoxStairs)
else:
    from .operators import kitfoxStairs
    from .operators import kitfoxStairsCurved

import bpy

Scripts can be reloaded through F3 &gt; Reload Scripts. Blender 2.8x doesn't have the F8 shortcut anymore, but you can add it to your keymap. Since Blender 2.83 this feature is available via the top bar menu Blender Icon &gt; System &gt; Reload Scripts.
Note: I've removed the if __name__ == &quot;__main__&quot;: from the __init__.py since this doesn't work with relative imports.

There is an error in the menu_func of kitfoxStairsCurved.py. It should reference the AddStairsCurved class instead of AddStairs. The correct function is:
def menu_func(self, context):
    self.layout.operator(AddStairsCurved.bl_idname, icon='MESH_CUBE')